In order to further investigate the lateral leakage loss behavior of shallowly-etched rib waveguides on silicon-on-insulator, a kind of shallowly-etched rib waveguide with non-rectangular crosssection was proposed and analyzed. A periodical loss model and a formula of loss period of this waveguide were obtained by the interference theory, and then phenomena of periodic variation of lateral leakage loss and shift of the maximum loss point for TM0-like mode were observed through the finite element method simulation in frequency domain with perfectly-matched layerconditions. The period in simulation results are highly consistent with theoretical calculation, and the average relative error is only 0.56%.In addition, it is found that TM0 mode loss can be adjusted from maximum to minimum by changing the cross section of rib waveguides with some trench widths, while TM0 mode loss is not sensitive to the change of cross section in rib waveguide with other trench widths. These findings can simplify process and improve fabrication tolerance, which provide theoretical guidance for the design and fabrication of this kind of waveguide.